The present invention relates to up converters which, from two radio-electric signals, allow a third radio-electric signal to be obtained whose frequency is higher than that of the first two signals. Such up converters are particularly useful for coherent Doppler radars in which all the signals are obtained from a local quartz oscillator whose frequency and phase are very stable.
In a particular embodiment of such a radar, as local oscillator a quartz is used whose frequency is 60 MHz. This frequency is relatively low, but it is difficult to cause a quartz to oscillate at a higher frequency, especially while maintaining its characteristics of precision and stability. So as to obtain the transmission signal it is then necessary to multiply the frequency of the signal supplied by the quartz. This multiplication is effected with the help of a whole conversion chain. In this chain we find particularly a mixer in which a signal at a frequency of 60 MHz is added to a signal at a frequency of 1800 MHz. The signal at the output of this mixer has then a frequency of 1860 MHz. This frequency is higher than that of the input signal, which is a rather unusual characteristic, and this is why this mixer is termed up converter.
The mixers commercially available may operate by stepping up the frequency. It has however been noted that their efficiency is then considerably inferior and is characterized for example by a drop of the output level greater than four decibels between the two operating modes. In addition, this relatively acceptable result is only obtained if the frequencies of the signals to be mixed are not too far apart from each other. This is clearly not the case in the mixer of the above-described radar.
It is usual for mixers operating up to frequencies of the order of 1500 MHz to form them by winding balanced transformers on magnetic cores such as ferrite beads or pots.
Beyond this frequency of 1500 MHz, most often a so-called microstrip hybrid technology is used comprising couplers or rings.
In the case of this radar, the lowest frequency justifies use of the wound technology and the highest frequency the hybrid technology. These two technologies are also called localized constant technology and spread constant technology. If a mixer is constructed with one of other of these technologies in this case, the losses will be too great in the windings on the ferrite for the 1800 MHz signal and the dimensions of the hybrid couplers will be prohibitive for the 60 MHz signal.